I find it strange how some fans would think Spike is so young when he has demonstrated maturity even beyond the 6 main ponies sometimes. See Lesson Zero and Best Night Ever. Sure he can be immature some times but so do the 6 main ponies from time to time.
Compared to the CmC's, Spike is responssibly independent like being on royal business to Canterlot for days on end as mentioned in Look Before you Sleep. I always imagined it was Spike that takes care of Twilight instead of the other way around like cooking or cleaning for her as implied in Its About Time when Twilight left Spike home alone to go to Tartarus. He is a good cook as shown or mentioned in episodes like Over a Barrel, Owls Well that Ends Well and A Bird in the Hoof. In Secret of my Excess, its implied Spike has no curfew because Twilight didn't wait for him to come home before going to bed.
I think why people consider him too young at first glance is because of his tiny dragon body. Pic related. Yes, he's a baby dragon but he is not a baby pony like the Cake twins. Yes, Twilight hatched his egg when she was young but we don't have a declaration of age or how long time has passed since then. Lauren Faust also said the ages were deliberately designed ambigous for story telling purposes. We don't know how these mythical creatures age and we don't know how long a day/year lasts on a world where the sun and moon are controlled by two alicorns.
I disagree withThereisnospoon303 saying the writers reined in regarding Spike and Rarity's ship tease due to real world reasons. S2 episodes ended production some time around late Dec 2011. If that's the case, by the time SomE and DQ aired, both episodes were already completely assembled and ready for airing. There's no way the writers could predict how the fans would react months in advance to what they write in their episodes.
Spike and Rarity are cute and the writers know this. The moment that you sexualize it is the only time it looks weird and that's why you shouldn't. Both characters are innocent and fairy tale-ish with their idea of romance. Rarity as a princess meeting her prince while Spike as a knight saving a princess.
And if Rarity only really likes Spike as a kid/brother/pet, wouldn't the writers make her tell some other pony about it to verbalize it on screen in an episode? Even if it was just for a gag. I mean, the show's real target audience are young children. Kids aren't gonna over analyze each character's behavior and create a subjective conclusion behind it. Kids don't even know what friendzone means. What they see on screen, they'll take it at face value and whatever that may be is what the writers intend to convey. I mean, take Spike's crush on Rarity, for example. The writers didn't beat the bush around that. Or the friendship reports at the end of each episode as a recap for the kids in case they didn't get it.
The way I see it, Spike and Rarity started out as a gag for the characters involved. An unrequitted secret crush used for visual humor. Then it changed by A Dog and Pony Show. The writers turned it into a story about this underdog boy doing touching heroic acts to win the heart of an unattainable girl. Its like a side story now simillar to how the CMC's do things to earn their cutie marks (and how RD wants to be a Wonderbolt member to a certain degree). But unlike the CMC's, Spike/Rarity side story progressed. By SomE, the secret crush was acknowledged. And by DQ, the girl's apathy has melted away and is finally noticing the boy. I can't wait to see what the writers have in store for Spike and Rarity in S3. There's no doubt they'll share at least 1 future episode in S3 like they did in the past two seasons.
Just my opinion.